Quantum Light Communication

Quantum Communication Underwater using hBN Single Photon Emitters

Challenges in Underwater Communication

• Acoustic waves: work over long distances but low data rates, insecure, omni-directional
• Electromagnetic waves (Radio, Infrared) cannot propagate underwater
• Optical wavelengths mostly absorbed → communication limited to few meters

Current Optical Communication Issues

• Blue/green light (~417 nm) reduces absorption
• Attenuated lasers used for underwater communication
• Probabilistic photon generation → not ideal for high-security applications
• Need for reliable, on-demand quantum light sources

Hexagonal Boron Nitride (hBN) Single Photon Emitters

• B-centres in hBN emit at 436 nm
• Engineered using electron beam
• Photostable and reliable
• Emission near water absorption minimum
• Suitable for underwater quantum communication

Transmission Through Water Channels

• Transmission & purity largely unaffected by short water paths
• Performs better than other wavelengths
• Demonstrated in Quantum Key Distribution (QKD) experiment
• Achieved kilobits per second rates
• Fully secure communication possible

Why This Matters

• Provides secure, high-speed underwater communication
• Practical quantum optical communication underwater
• Establishes spectral window (~436 nm) for applications
• Potential uses: submersibles, research, military operations

Key Takeaways

• Acoustic waves = slow & insecure; standard EM blocked underwater
• hBN B-centres at 436 nm = ideal photon source
• Maintains high transmission & purity underwater
• Enables secure Quantum Key Distribution

References

• Include key references from your research paper

Quantum Communication Underwater: Key Findings and Applications

1. Problem: Underwater Communication Challenges

Underwater communication is a growing need, especially for research, military, and data transfer in submersibles, but current methods are problematic.

• Acoustic waves, commonly used underwater, offer long-range but suffer from low data rates, low security, and are omnidirectional (unsecure).
• Electromagnetic waves (radio, infrared) cannot be used effectively underwater because water absorbs almost all wavelengths, except in the blue spectral range (~417 nm).
• Traditional optical methods don't offer the necessary security (because light waves behave probabilistically), so quantum communication is explored to solve this.

2. Solution: Hexagonal Boron Nitride (hBN) and Single-Photon Emitters

hBN is identified as a material that can support point defects (B-centres) which emit single photons at 436 nm—wavelengths near the absorption minimum in water (~417 nm).

• This makes hBN a promising candidate for secure quantum communication, as single-photon emission (SPE) guarantees security.
• These B-centres can be engineered using an electron beam and have extreme photostability, making them ideal for this application.

3. Experimental Setup

The study involved measuring the photoluminescence (PL) of B-centres using a confocal microscope with a 1 m water path.

• Emission spectra of B-centres show negligible transmission loss through water compared to other wavelengths (e.g., red photons at 620 nm).
• The experiment demonstrates that B-centres maintain single-photon purity after traveling through the water, confirmed using the second-order autocorrelation function.

4. Key Findings

Minimal Loss Through Water

The B-centre emission at 436 nm shows minimal loss over water distances (even up to 3 meters) compared to other wavelengths (like red photons at 620 nm).

• Water’s absorption coefficient is much lower for blue light, minimizing loss and enabling communication at greater distances.

Quantum Key Distribution (QKD)

The QKD experiment (a method for secure communication based on quantum mechanics) was successfully demonstrated using the B-centres.

• A QKD setup was used with Alice (the sender) and Bob (the receiver), transmitting quantum-encrypted keys through a 1 m water channel.
• The experiment yielded a quantum bit error rate (QBER) of 0.09, well within the threshold for secure QKD, proving that the underwater transmission of quantum-encrypted keys works effectively.

Comparison of Emission Wavelengths

The study also compared the B-centre (436 nm) to a red emitter (620 nm). The red emitter showed much higher absorption losses in water, further confirming the advantage of blue-emitting B-centres.

5. Potential Impact and Applications

Quantum Communication

This research highlights a significant leap in underwater quantum communication, enabling secure data transfer at high speeds with minimal transmission loss.

Security for Military and Research

The ability to securely transfer data using quantum key distribution (QKD) underwater is a game-changer for military and sensitive scientific operations, where traditional encryption methods are vulnerable to attacks, especially with the rise of quantum computing.

6. Challenges and Future Work

EOM Performance

The extinction ratio of the Electro-Optic Modulator (EOM), which is key to creating polarization states for QKD, limited the transmission rates. The QKD rate was affected by the modulator's inefficiency, leading to a QBER of 0.09. However, the paper states that future improvements in photodetectors, modulator efficiency, and water filtering could overcome these limitations.

Collection Efficiency

The collection efficiency was low due to the low quantum efficiency of detectors at 436 nm. Using improved detectors and cavity engineering could boost the emission rate.

Real-world Deployment

The setup’s practicality for long-distance underwater communication depends on optimizing the optical setup and improving quantum emitter performance.

Conclusion

The study demonstrates the feasibility of using B-centres in hBN for quantum communication underwater.

• Secure QKD was successfully achieved over a 1-meter water channel, confirming that quantum communication in the blue spectral range (436 nm) is viable for real-world underwater applications.
• In summary, the paper shows that hBN-based quantum emitters offer a promising solution for high-speed, secure underwater communication, with applications in military, research, and defense industries.
• Despite challenges in modulator efficiency and detector performance, the work paves the way for the development of secure quantum communications in underwater environments.

Single-Photon Production and Detection in Quantum Communication

Great question! Let's break down how single photons are produced and received in a typical quantum communication setup, particularly in the context of the research paper where hexagonal boron nitride (hBN) is used as the single-photon emitter.

How Single Photons Are Produced

1. Single Photon Emission (SPE)

• A single photon emitter (SPE) is a device or material designed to emit one photon at a time.
• In this research, the emitters are point defects (called B-centres) inside hexagonal boron nitride (hBN).
• These point defects are created by electron beam irradiation.

Electron Beam Irradiation: An electron beam with a controlled energy and dose is directed at the hBN crystal. This creates defects in the crystal lattice (B-centres) that can emit single photons when excited.

2. Excitation of the Emitters

• The single-photon emitters are excited using an external light source.
• Typically, a 405 nm laser is used.
• The laser excites electrons in the B-centres from a lower energy state to a higher energy state.
• When the electron relaxes back to the ground state, it emits one photon.

3. Photon Emission

• The emitted photon has a specific wavelength—around 436 nm (blue light).
• Only one photon is emitted per excitation cycle.
• This one-to-one emission is critical for quantum security, since each bit of information is carried by exactly one photon.

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How the Single Photon Is Precisely Received

After emission, the photon must be collected and detected with high precision. This is essential for applications like Quantum Key Distribution (QKD).

1. Photon Collection

• The photon propagates through space or through a medium such as water.
• Optical elements like lenses or a confocal microscope collect and focus the photon.
• In underwater experiments, photons travel through a sealed water-filled tube to simulate underwater communication.

2. Photon Detection

Once collected, the photon is directed to a detector capable of sensing individual photons.

• Avalanche Photodiodes (APDs):
  • Extremely sensitive detectors capable of detecting single photons.
  • A photon triggers an avalanche of charge carriers, producing a detectable electrical signal.
  • The experiment uses an Excelitas SPCM-AQRH, optimized for blue wavelengths (~436 nm).
• Single-Photon Counters (SPCs):
  • Devices that count individual photon detection events.
  • Convert photon arrivals into electrical pulses.
• Hanbury Brown–Twiss (HBT) Interferometer:
  • Used to verify that the source emits single photons.
  • Measures the second-order correlation function g²(0).
  • For true single-photon sources, g²(0) is much less than 1.

3. Quantum Key Distribution (QKD) Setup

• The detector is part of a QKD system.
• Alice (sender) encodes information onto photons using polarization.
• Bob (receiver) measures the photons to decode the key.
• Key optical components include:
  • Electro-Optic Modulators (EOMs)
  • Polarizing Beam Splitters (PBS)

4. Measurement and Analysis

• Detected signals are analyzed electronically.
• The second-order autocorrelation function g²(0) is calculated.
• This confirms whether the detected light is truly single-photon emission or background noise.

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Summary of the Instruments Involved

1. Excitation Source: 405 nm laser
2. Confocal Microscope: Collects and focuses emitted photons
3. Photon Detectors: Avalanche Photodiodes (APDs) or Single-Photon Counters
4. HBT Setup: Verifies single-photon purity
5. QKD Optical Components: EOMs and PBS for polarization encoding

By combining these instruments, researchers can reliably generate, transmit, and detect single photons, making secure, high-speed quantum communication possible—even in underwater environments.